Pyrolytic carbon used to coat parts of heart valve prostheses is formed as a coating on substrates in high temperature deposition processes. A fluidized bed reactor is generally used for the coating operation. In the deposition process a gas stream, consisting of a mixture of hydrocarbon gas (e. g. propane, acetylene, methane, propylene or the like) and an inert gas (e. g. helium, argon, nitrogen or the like), is metered through a vertical heated graphite chamber containing a bed of spherical particles or beads. The shape of the graphite reactor chamber at the gas entrance is generally conical. The gas stream fluidizes or levitates the bed of particles along with valve component substrates suspended within the bed and a part thereof. Pyrolytic carbon coating apparatii of this kind are disclosed in U.S. Pat. Nos. 5,262,104; 5,305,554; 5,332,337; 5,328,720; 5,284,676; 5,328,713; 5,514,410; 3,676,179; 3,677,795 and 3,977,896, each of which is incorporated herein by reference.
The pyrolytic carbon used in heart valves is a form of high density, high wear resistant, high strength carbon. From the biomedical point of view it is noteworthy because of its biocompatibility. From scientist""s perspective it is a high technology material which is prepared as a coating on substrates in a fluidized bed and elevated temperature deposition process. In the deposition process a suitable hydrocarbon gas (e. g. propane ) along with neutral gas (e. g. helium, argon and/or nitrogen) is metered through a vertical heated graphite chamber containing a bed of granular particles. The shape of the graphite chamber at the entrance is generally conical. This gas stream fluidizes, i. e. levitates and agitates the bed of particles along with any valve substrate components suspended in the bed. The fluidized bed is generally heated in one manner or another, preferably by furnace. The heated bed, in turn, heats the gases. When sufficiently hot, the reactant gases, such as propane and/or other hydrocarbon gases, pyrolyze, thereby dehydrogenating, and form pyrolytic carbon as a solid coating. The pyrolytic carbon coating deposits on the suspended valve components as well as the bed of granular particles. The action of the fluidized bed causes the valve components to continuously circulate through out the bed resulting in coating over their entire surface.
According to the disclosure provided in U.S. Pat. No. 3,977,896 to Bokros et al. (Bokros ""896 patent), one of the key parameters which determine the structure of pyrolytic carbon, is the ratio of available deposition surface area relative to the volume of hot zone occupied by the fluidized bed of particles. In the ""896 patent disclosure, this parameter is used as a variable for characterizing carbon coating processes. It is believed by the present inventor, that pyrolytic carbon deposited on levitated valve components has certain desirable properties when the surface area of the particles is fairly high when compared to the surface area of the valve components. When such sub-millimeter particles are being coated along with valve components in a fluidized bed, the total surface area of the particles begins to increase significantly as the diameters of the particles increase. However, because of the substantial growth of surface area of the particles, most of the carbon deposition is deposited on the particles rather on the valve components. The coating thickness thus obtained on the valve components is fairly limited. The coating thickness required for manufacture of heart valves is generally higher than possible via the process described above. In order to overcome this difficulty, researchers developed a technique in which small size particles were fed to the hot zone and large coated particles were removed from the coating chamber during the deposition process without causing undue disruption to the process. The Bokros ""896 patent is probably the first patent that disclosed the concept of feeding the small size beads and withdrawing the larger coated beads during the deposition process. This technique made it possible to achieve thicker coatings required for heart valve applications.
The Bokros ""896 patent describes a number of process parameters for carrying out coating processes. A detail review of specific items pertinent to the fluidized coating reactor apparatus and the process parameters disclosed therein is provided below.
Particle Feed and Purge Systems
As mentioned above, the Bokros ""896 patent discloses an apparatus for carrying out coating processes. The patent mentions process parameters for depositing coatings having desirable properties, and feed and withdrawal systems for achieving relatively thick (at least 150 micrometers or microns) coatings. Coating thicknesses greater than 150 microns are believed to be possible by lengthening the duration of coating period. The Bokros ""896 patent discloses the idea, the apparatus, and a method of feed and withdrawal of particles. The particle withdrawal is carried out by utilizing a small diameter tube that is placed alongside a wall within the deposition chamber. A neutral gas is continually passed upward through this tube. By adjusting the upward flow rate of neutral gas, coated particles of varying quantities are removed from the fluidized bed. The height of the tube is less than 5 cm above the conical section of the chamber. The internal diameter of the withdrawal tube is such that a helium gas flow of 4 liters per minute prevents removal any particles from the fluidized bed. The withdrawn particles are volumetrically metered. By adjusting the upward flow rate of neutral gas through the withdrawal tube, a purge rate at 90 to 150 cubic centimeters per hour can be achieved. The feed particles are fed via a similar size tube placed along the cylindrical wall of the chamber. The particles are fed intermittently or continuously near the fluidized zone from the top. The feed tube is continually purged with sufficient downward flow of neutral gas.
Source Materials
The Bokros ""896 patent discloses several hydrocarbon gases such as methane, propane, ethane, butane, acetylene and propylene for depositing pyrolytic carbon coating. Helium, argon and nitrogen gas are used as neutral gases. The examples given describe use of propane and helium gases for depositing pure and silicone alloyed pyrolytic carbons.
In each of these systems, the pyrolytic carbon comes from the propane or other hydrocarbon gas which enters the reactor and is deposited on the valve component substrates by an endothermic chemical decomposition reaction described for example, in the systems employing propane, by the reaction provided immediately below:
C3H8(g)298xc2x0 Kxe2x86x923C(s)+4H2(g), xcex94HR=24.82 Kcal/gmole
The reactor chamber is heated by an electrical heating element which in turn heats the gases. When the gases are sufficiently hot, the hydrocarbon gas breaks up or decomposes by a process called pyrolysis to form a pyrolytic carbon coating on the beads and on the valve component substrates, and gaseous hydrogen. The helium or other inert gas serves as an inert carrier/fluidizing gas. The inert, or neutral, gas does not react in the decomposition reaction. The pyrolytic carbon coating is deposited on the suspended valve component substrates, as well as on the beads, under suitable conditions. The action of the fluidized bed generally causes the valve components to circulate through the entire bed, resulting in coating over all exposed surfaces.
In the reactors presently used to coat components of heart valves, the gas stream from the gas inlet into the reactor is believed to be a steady stream of gas. The stream of gas may be metered, but it will still be a steady stream having a constant flow. At times, such a system will malfunction when two or more of component parts become engaged with one another against the inner wall of the reactor, thereby disrupting the bed by immobilizing a portion of it, and thereby changing the coating parameters of the xe2x80x9crunxe2x80x9d such that a consistent coating process cannot be maintained.
Furthermore, in order to obtain consistent coating on valve components or the like coated in consecutive or non-consecutive batch operations, strict attention must be paid to keeping all parameters exactly the same. This is a difficult task at best. Indeed, it is believed that consistent coating in pyrolytic coating processes cannot be obtained without at least some variance from batch to batch with the prior art coating apparatus described in the Bokros ""896 patent, or those generally used at present for batch process coating processes. This is because each reactor has its own irregular frequencies which change with the time of the run as the suspended substrates and beads are coated. New uncoated beads are metered into the bed and coated beads are removed from the bed in order to maintain a generally consistent average weight and density for the beads, but the weight and density of the substrates increase throughout the run. This results in alterations in fluidized bed action within the reactor, and leads to undesirable inconsistencies in the coating deposited on the various substrates during the run.
Accordingly, it will be appreciated that there is a need for an efficient pyrolytic carbon coating apparatus or system for providing a consistent coating of pyrolytic carbon on valve components and the like. The present invention provides advantages over the prior devices and prior methods used to coat valve components, and also offers other advantages over the prior art and solves other problems associated therewith.
The pyrolytic carbon coating apparatus of the present intention includes a fluidized bed reactor having a reactor chamber, a gas feed inlet, and an exhaust gas outlet, a source of process feed gas, preferably including a mixture of gaseous hydrocarbons and inert carrier gases, more perferably including gases selected from the group consisting of gaseous hydrocarbons, inert carrier gases and mixtures thereof, and a gas line through which the process feed gas can pass from the source of process feed gas to the gas feed inlet and into the reactor chamber. The fluidized bed reactor is of a type which generally permits the levitation or fluidization of substrates in a pyrolytic carbon coating environment where gaseous hydrocarbons within the process feed gas is decomposed at elevated temperatures in order to coat surfaces of fluidized substrates with pyrolytic carbon. The gas line of the present invention includes an actuator which varies the rate of flow of process feed gas into the reactor chamber, such that the flow of process feed gas, through the gas line to the reactor chamber when the reactor chamber is occupied by a fluidized bed including at least one or preferably plurality of substrates to be coated by pyrolytic carbon, cycles regularly over a consistent period of time from a higher flow rate to a lower flow rate and vice versa, so as to create a pulsating gas flow and a pulsation effect upon the fluidized bed within the reactor chamber. The preferred actuator includes a bypass line and a main line through which process gas can flow. The bypass line includes a metering valve, preferably a needle valve, which can be used to restrict the flow of process gas through the bypass line. The main line includes a gas filter, preferably a high purity gas filter, through which the process gas flows prior to flowing through a switch valve, preferably an oscillating solenoid valve, which opens and closes in a regular cycle thereby alternately stopping and permitting the flow of process gas through the main line. The bypass line and the main line both diverge from and subsequently rejoin the gas line to direct a pulsating flow of process feed gas through the gas line into the reactor chamber having a flow rate which regularly cycles from a higher flow rate to a lower flow rate and vice versa. The apparatus enhances the ability of the user to provide a consistent coating of pyrolytic carbon on different sizes and types of component substrates. This is particularly desirable for individual components of heart valve prostheses.
The present pyrolytic carbon coating apparatus allows one to control the frequency and amplitude of the pulsating feed gas flow rate. As the needle valve in the bypass line is moved toward a closed position, thereby decreasing the rate of flow through the bypass line, the difference between the higher flow rate and the lower flow rate increases. As the needle valve in the bypass line is opened, thereby increasing the rate of flow through the bypass line, the difference between the higher flow rate and the lower flow rate decreases. The frequency and the amplitude can be controlled in a manner which allows this use to produce coated component parts having consistent coating parameters from one batch to another. A sufficient flow of steady process gas through the bypass line is maintained so as to avoid the collapse of the fluidized bed.
It is an object of the present invention to provide a reactor in which gaseous hydrocarbons are efficiently decomposed to form a pyrolytic coating on substrate surfaces which is of greater quality than that which has been previously available, and which can be applied to the substrate surfaces more evenly and consistently over a series of batch coating processes, while creating less soot buildup within the reactor chamber.